The present invention relates to an apparatus for diagnosing malfunctioning in an oxygen concentration sensor which is provided downstream from a catalyst in the exhaust passage of an internal combustion engine.
In internal combustion engines, exhaust components are purified by a catalyst for purifying exhaust, which is provided in the exhaust passage. This purification of the exhaust components by the catalyst is carried out efficiently in the case where the air-fuel ratio of the air-fuel mixture, which is burned in the internal combustion engine, is within a predetermined range. Therefore, an air-fuel ratio feedback control is generally carried out such that an oxygen concentration sensor for detecting the concentration of oxygen in the exhaust is provided in a section of the exhaust passage on the upstream side of the catalyst, the air-fuel ratio of the air-fuel mixture is detected on the basis of an output signal of this sensor, and the correction value of the air-fuel ratio relative to the amount of fuel injection is determined so that this detected air-fuel ratio becomes the target air-fuel ratio, and thus, the amount of fuel injection is increased or decreased.
In addition, to obtain the state of the exhaust components purified by the catalyst, a “sub-feedback control” of the air-fuel ratio is also carried out in which an oxygen concentration sensor (downstream oxygen concentration sensor) is provided in a section of the exhaust passage on the downstream side of the catalyst, the air-fuel ratio of the exhaust after it has passed through the catalyst is detected on the basis of an output signal of this sensor, and a modification value relative to the above described correction value of the air-fuel ratio is calculated.
In the above described sub-feedback control, the output value of the downstream oxygen concentration sensor is used, and therefore, in the case where malfunctioning occurs in this oxygen concentration sensor, normal control cannot be maintained, which in turn leads to a risk that the exhaust may not be sufficiently purified. Therefore, during fuel cutoff, in which fresh air is introduced into the exhaust passage when fuel injection from the fuel injection valve is stopped, diagnosis of malfunctioning is carried out in which the downstream oxygen concentration sensor detects fresh air after the start of the fuel cutoff, and thus, the period until the output value of this indicates a lean air-fuel ratio is measured, and malfunctioning of the downstream oxygen concentration sensor is diagnosed on the basis of the thus measured time.
The catalyst provided in a section of the exhaust passage on the upstream side of the downstream oxygen concentration sensor has an oxygen storing function in so that it stores and releases oxygen. Therefore, when malfunctioning is diagnosed in the downstream oxygen concentration sensor on the basis of the period from the start of the fuel cutoff until the output value of the downstream oxygen concentration sensor indicates that the air-fuel ratio is lean, this period fluctuates when affected by the oxygen storing function, and thus, the accuracy in detecting malfunctioning decreases.
In order to prevent the detecting accuracy from decreasing due to this oxygen storing function, in the apparatus described in, for example, Japanese Laid-Open Patent Publication No. 2003-343339, malfunctioning is diagnosed in the downstream oxygen concentration sensor on the basis of a change in the output value within a predetermined period of time after fuel cutoff has been carried out and the output value of the downstream oxygen concentration sensor has actually started changing towards a side representing lean air-fuel ratio (lean side). According to this diagnosis of malfunctioning, malfunctioning can be diagnosed on the basis of only the responsiveness of the downstream oxygen concentration sensor.
Incidentally, the output value of the downstream oxygen concentration sensor sometimes gradually changes towards the lean side due to the oxygen storing function of the catalyst even before fresh air reaches the downstream oxygen concentration sensor.
Therefore, as described in Japanese Laid-Open Patent Publication No. 2003-343339, it is determined that the responsiveness of the downstream oxygen concentration sensor is low in the case where malfunctioning is diagnosed in the downstream oxygen concentration sensor on the basis of a change in the output value within a predetermined period of time after the output value of the downstream oxygen concentration sensor has started changing towards the lean side and when a change in the output value occurs as described above before the above described fresh air arrives at the downstream oxygen concentration sensor. As a result, the downstream oxygen concentration sensor may be erroneously determined to be malfunctioning.